C.O.6 Complete genetic analysis by whole exome sequencing of a cohort with centronuclear myopathy identifies titin gene mutations

Abstract

Centronuclear myopathies (CNM) are a group of rare heterogeneous congenital myopathies characterized by muscle weakness and low tone of varying onset, often requiring respiratory, feeding and/or ambulatory assistance. Frequent central nuclei are the primary pathological finding in affected skeletal muscles. CNM may be caused by mutations in the MTM1, DNM2, RYR1 and BIN1 genes. MTM1 mutations, which cause X-linked myotubular myopathy, are thought to account for about 50% of CNM cases, while DNM2 and RYR1 account for ∼15% each, and BIN1 for <5%, leaving about ∼15–20% of cases without a known disease gene. To identify known and novel gene mutations responsible for CNM, we performed whole exome sequencing (WES) in 28 probands with sporadic CNM from a population already depleted by clinical testing for MTM1 and DNM2 mutations. RYR1 and BIN1 had largely not been sequenced because of the large gene size (RYR1) and rarity (BIN1) of these mutations. WES was performed by Axeq Technologies using the Illumina HiSeq 2000 platform, which generated 10× coverage of 84–88% across the target regions in these samples. Concomitantly, one of these 28 patients was identified to have MTM1 duplication on further clinical testing. Of the remaining 27, two had DNM2 and nine had RYR1 mutations that were considered probably pathogenic. Remarkably, we identified five cases with compound heterozygote mutations of the titin gene (TTN), the majority being frameshift insertions, deletions, or stopgain mutations. Analysis of the remaining cases has identified at least one additional new candidate gene, and further testing is underway. In summary, exome sequencing has identified the molecular basis for roughly 60% of these patients with CNM. Work is underway to identify novel genes in the remaining 11 exomes.